1. Field of the Invention The present invention relates to apparatus for measuring rotation. More particularly, this invention pertains to a fiber optic Sagnac interferometer for measuring rotation rates about three axes in space that requires only a single source of optical energy and a single photodetector.
2. Description of the Prior Art
The Sagnac interferometer is an instrument for determining rotation rate through measurement of the nonreciprocal phase difference generated between a pair of counterpropagating light waves. This instrument generally comprises a light source such as a laser, an optical waveguide consisting of several mirrors or a plurality of turns of optical fiber, a beam splitter/combiner, a detector and a signal processor.
In an interferometer, the waves leaving the beam splitter counterpropagate along a common optical path. The optical waveguide is "reciprocal"; that is, any distortion of the optical path affects the counterpropagating beams similarly although they do not necessarily experience such perturbation at the same time or in the same direction. Time-varying perturbations may be observed where the time interval is comparable to the propagation time of the light around the optical waveguide. These "nonreciprocal" perturbations affect the counterpropagating beams differently and according to the direction of propagation. Such nonreciprocal perturbations are occasioned by physical effects that disrupt the symmetry of the optical medium in which the two waves propagate.
Two of the nonreciprocal effects are quite well known. The Faraday, or collinear magneto-optic effect, occurs when a magnetic field creates a preferential spin orientation of the electrons in an optical material whereas the Sagnac, or inertial relativistic effect, occurs when rotation of the interferometer with respect to an inertial frame breaks the symmetry of propagation time. Such latter effect is utilized as the principle of operation of a ring gyroscope.
Many applications, including navigation, require rotation and position information with respect to the three orthogonal space axes. Accordingly, a triad of interferometers may be required, one for sensing rotation about each of the rotation axes. In the past, systems of this type have employed three independent interferometers. That is, each interferometer has utilized a single dedicated source of optical energy and a single photodetector. As a result, a total of three sources of optical energy and three photodetectors have been required. The use of multiple sources of optical energy and photodetectors adds significantly to the weight, power consumption, heat dissipation and cost of an overall navigation system. Also, a control circuit is required for each source, further adding to size, heat and power consumption problems.